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Interpretive Summary: Chitosan is the deacetylated form of chitin, which is the second most abundant polysaccharide on the planet and the main component of fungal cell walls, insect exoskeletons and crustacean shells. Chitosan has become of interest as a crop biostimulant suitable for use in sustainable agriculture since it is biocompatible, biodegradable, environmentally friendly and readily available in large quantity. Short-term (35-days after transplanting) effects of chitosan, applied as a soil amendment at 0%, 0.05%, 0.10%, 0.15%, 0.20% or 0.30% by weight, on lettuce (Lactuca sativa) growth, chlorophyll content, and photosynthesis were evaluated in a growth chamber study. Chitosan at 0.05%, 0.10% and 0.15% significantly increased leaf area from 674 to 856, 847 and 856 cm2, leaf fresh weight from 28.6 to 39.4, 39.1 and 39.8 g, respectively. Only chitosan at 0.05% and 0.10% significantly increased leaf dry weight from 3.42 to 4.37 and 4.35 g, while chitosan at 0.30% significantly decreased leaf number, area, fresh weight and dry weight. Chitosan at 0.10%, 0.15%, 0.20% and 0.30% significantly increased leaf chlorophyll content. Leaf photosynthesis rate also increased with chitosan at 0.15%, 0.20% and 0.30%, respectively. The results indicated that chitosan, at appropriate application rates, enhanced lettuce growth, and might have potential to be utilized for sustainable production of lettuce.

Technical Abstract: Chitosan has become of interest as a crop biostimulant suitable for use in sustainable agriculture since it is biocompatible, biodegradable, environmentally friendly and readily available in large quantity. Short-term (35-days after transplanting) effects of chitosan, applied as a soil amendment at 0%, 0.05%, 0.10%, 0.15%, 0.20% or 0.30% (w/w), on lettuce (Lactuca sativa) growth, chlorophyll content and fluorescence, and gas exchange were evaluated in a growth chamber study. Chitosan at 0.05%, 0.10% and 0.15% significantly increased leaf area from 674 to 856, 847 and 856 cm2, leaf fresh weight from 28.6 to 39.4, 39.1 and 39.8 g, respectively. Only chitosan at 0.05% and 0.10% significantly increased leaf dry weight from 3.42 to 4.37 and 4.35 g, while chitosan at 0.30% significantly decreased leaf number, area, FW and DW. Chitosan at 0.10%, 0.15%, 0.20% and 0.30% significantly increased leaf chlorophyll index from 29.8 to 34.4, 35.4, 37.5 and 41.4. Chitosan at 0.20% and 0.30% significantly increased leaf maximum photochemical efficiency (Fv/Fm) and photochemical yield (Y(II)), and chitosan at 0.10%, 0.15% 0.20% and 0.30% significantly increased leaf electronic transport rate. Leaf photosynthesis rate and stomata conductance increased from 9.3 to 12.7, 14.0 and 16.6 µmol CO2/m2/s, and from 0.134 to 0.183, 0.196 and 0.231 mol/m2/s, with chitosan at 0.15%, 0.20% and 0.30%, respectively. The results indicated that chitosan, at appropriate application rates, enhanced lettuce growth, and might have potential to be utilized for sustainable production of lettuce.